This invention relates generally to pressurized gas-metal cells such as nickel-hydrogen cells and, more particularly, to a process for reducing the open-circuit capacity loss during storage of such cells.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as a useful current. A familiar example of the rechargeable cell is the lead-acid cell used in automobiles. Another type of cell having a greater storage capacity per unit weight is the pressurized gas-metal cell, an important type of which is the nickel-hydrogen cell used in spacecraft applications. A nickel-hydrogen cell used in a satellite is periodically charged by electrical current produced by solar panels on the spacecraft, and then later discharged to supply electrical power, as when the spacecraft is in shadow or peak electrical power is demanded.
The primary requirements of cells to be used in spacecraft are high energy storage capacity per unit weight of cell, reliability, and the ability to be recycled through many cycles of charging and discharging. A newly assembled nickel-hydrogen cell has a high energy storage capacity as measured in ampere-hours. However, it is observed by many users that, after the cell has been stored in an open circuit or short circuit, discharged condition for a period of time, the maximum storage capacity upon attempting to fully charge the cell is reduced, as compared with the value that is attainable with a newly assembled cell. The loss of full-charge energy capacity that is experienced while the cell is stored gradually increases with increasing storage time, and can be as great as 20% of capacity after a storage time of one month. Once the cell capacity is lost in this manner, it is difficult or impossible to restore the cell fully to its initial high level of capacity.
This reduction of the electrical capacity of the cell during open-circuit storage presents a serious problem for the manufacturer and the user of the cell. It is usually most efficient to manufacture and assemble the cell at a site different from that of its installation into the spacecraft. The cell must therefore be transported after assembly. Moreover, installation of the cell into the spacecraft may occur several days or weeks prior to the actual launching of the spacecraft and charging of the cell by the solar panels, because of the need to check the spacecraft with the cells in place and also because of possible delays in the launching of the spacecraft. The time between the final assembly of the cell and the actual first operation of the cell in space is often as much as one year, with the result that the electrical storage capacity of the nickel-hydrogen cell may be substantially reduced. Moreover, it is usually desirable to manufacture a number of cells at one time, so that the actual storage time may be even longer, with a corresponding greater decrease in the capacity of the cell when it is finally placed into service.
It is not practical to delay the final assembly and activation of the cells until just before the launch of the spacecraft, because this procedure would be inefficient, cumbersome, and would significantly interfere with the smooth scheduling of the launch procedure of the spacecraft. It is also not practical to ship the cells partially charged, due to the hazards arising from the presence of pressurized hydrogen. For these reasons, the various approaches to reducing or eliminating the reduction in cell capacity during cell storage either have not been successful or are excessively costly.
A need therefore exists for an approach for reducing or eliminating completely the loss in electrical charge capacity of nickel-hydrogen cells. The approach should be fully capatible with existing cell components and handling procedures, and should not necessitate significant modifications to spacecraft launching procedures. The approach should not require specialized, complex or expensive equipment that would be used in conjunction with the launching of the spacecraft, since such additional equipment would complicate the launch procedures and possibly create difficulties because of the support equipment. The approach should also not significantly increase the weight of the cell or decrease its charge capacity, charging and discharging characteristics, or ability to be charged and discharged thousands of times during the operating life of the spacecraft. The present invention fulfills this need, and further provides related advantages.